Organic thin film transistors, which are drive elements for next-generation displays, are being researched actively. Organic thin film transistors (OTFTs), which are prepared using an organic film instead of a silicon film as a semiconductor layer, are classified into low-molecular organic thin film transistors, such as oligothiophene, pentacene and the like, and high-molecular organic thin film transistors, such as polythiophenes and the like, according to the material used for the organic film.
An organic electroluminescence display, which uses such organic thin film transistors as a switching device, is configured such that a plurality of pixels is arranged on a substrate in a matrix form in which each pixel includes two organic thin film transistors, for example, a switching organic thin film transistor and a driving organic thin film transistor, a capacitor, and an organic light emitting device composed of an organic film interposed between upper and lower electrodes.
Generally, a flexible organic electroluminescence display uses a flexible substrate, and the flexible substrate includes a plastic substrate. Since the plastic substrate has very low thermal stability, it is required to manufacture an organic electroluminescence display through a low-temperature process.
Therefore, since organic thin film transistors, each of which uses an organic film as a semiconductor layer, can be used in a low-temperature process, they are attracting considerable attention as a switching device for a flexible organic electroluminescence display.
Korean Unexamined Patent Application Publication No. 2004-0028010 discloses a pentacene thin film transistor which can shorten a thin film deposition time and improve hole mobility. Further, Korean Unexamined Patent Application Publication No. 2004-0084427 discloses an organic thin film transistor which can improve the electric performance of transistors and a method of manufacturing the same. Furthermore, Japanese Unexamined Patent Application Publication No. 2003-92410 discloses an organic thin film transistor which can improve carrier mobility and ON/OFF current ratio because its channel region is composed of organic compounds having radicals.
However, since the above methods of manufacturing an organic thin film transistor are disadvantageous in that the manufacturing cost is high, methods of manufacturing an organic thin film transistor at low cost has been required. For this reason, ink-jet printing technologies have attracted considerable attention as direct patterning methods required to manufacture organic electronic devices, such as organic light emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic solar cells (OSCs), radio-frequency identifications (RFIDs) and the like, at low cost.
International Patent Publication No. 1999-39373 (WO 1999/39373) discloses a method of forming patterns on a substrate using ink-jet printing, in which an organic material, such as an organic semiconductor, dissolved in a solvent, such as chloroform, is deposited on the substrate, and then the solvent is volatilized to leave the organic material on the substrate.
However, the organic semiconductor layer formed using ink-jet printing is problematic in that it has low performance compared to organic semiconductor layers formed using spin coating or deposition because it is formed on the surface thereof with nonuniform morphologies and has low crystallinity.
In order to solve the above problem, novel organic semiconductor materials, such as poly(alkylthiophene) precursors, oligothiophene precursors, pentacene precursors and the like, are being developed, and methods of improving printing factors, such as the kinds of solvents for organic semiconductors, the surface wettability of a substrate, the concentration of a solution and the like, are being researched (M. Plotner, T. Wegener, S. Richter, W. J. Fischer, Synthe. Met. 2004, 147, 299., S. K. Volkman, S. Moleda, B. Mattix, P. C. Chang, V. Subramanian, Mater. Res. Soc. Symp. Proc. 2003. 771, 391., P. C. Chang, S. E. Molesa, A. R. Murphy, J. M. J. Frechet, V. Subramanian, IEEE Trans. Electron Devices 2006, 53, 594).
However, there are also unsolvable problems in that it is difficult to form uniform morphologies due to the coffee stain effect occurring during a process of drying a solvent and in that crystallinity cannot be exhibited even though uniform morphologies are formed.
Therefore, methods of controlling a drying process in order to form a uniform organic semiconductor layer after the drying process are continuously required. In particular, in the case of low molecular organic semiconductors, methods of manufacturing an organic semiconductor layer having crystallinity after the drying process are also required continuously.